Intake and/or exhaust gas sensors may be operated to provide indications of various intake and exhaust gas constituents. Output from a Universal Exhaust Gas Oxygen (UEGO) sensor, for example, may be used to determine the air-fuel ratio (AFR) of exhaust gas. Similarly, an oxygen sensor may be disposed in an engine intake passage to determine the humidity of intake gas or the composition of exhaust gases being recirculated to the intake. Indications of intake and exhaust gas oxygen content may be used to adjust various engine operating parameters, such as fueling. As an example, cylinder fueling may be feedback controlled based on exhaust gas AFR to achieve a target combustion AFR that maximizes the operating efficiency of an exhaust emission control device. As such, the measurement accuracy of an oxygen sensor may be significantly affected by degradation of an element in the oxygen sensor, such as due to sensor element blackening or thermal aging. Oxygen sensor element blackening is a form of degradation which may occur due to application of high and/or variable voltage and due to low oxygen and water conditions. Thermal aging is another form of degradation that occurs due to continuous operation of the sensor over a period of time.
Various approaches have been used to detect oxygen sensor degradation especially in relation to blackening and thermal aging. In one example approach, shown by Verdier et al. in US 20120167656 A1, a voltage may be applied to terminals connected to electrodes of a pump cell (in the case of a dual-cell) sensor or that of a combined pump and measuring cell (in the case of a single-cell sensor) of the exhaust gas sensor and a pumping current flowing through the cell is subsequently monitored. Voltages of equal magnitudes may be applied to the terminals at different points of time and the difference in pumping current is estimated. If the difference in pumping current is higher than a threshold value, oxygen sensor degradation due to blackening of at least one of the cell electrodes may be inferred.
The inventors herein have recognized potential issues with the above mentioned approach. As one example, following an application of voltage at two different points of time, a change in pumping current may occur due to thermal aging or element blackening. Consequently, in the approach of Verdier et al., it may not possible to differentiate between thermal aging of the sensor and sensor element blackening. Based on the occurrence of thermal aging and/or element blackening, different corrective measures for future oxygen estimations using the sensor are required to be taken. By applying inappropriate corrective measures to UEGO sensors, sensor degradation may be accelerated and the accuracy of oxygen estimation using the sensor may decrease thereby affecting engine performance.
The inventors herein have identified an approach by which the issue described above may be at least partly addressed. One example method for a vehicle engine comprises differentiating between thermal aging and blackening of an oxygen sensor element based on a monitored change in impedance of each of a pump cell and a Nernst cell of the oxygen sensor following application of a voltage. In this way, oxygen sensor blackening can be identified more reliably and promptly addressed.
As an example, impedance of each of a pump and a Nernst cell of an oxygen sensor may be monitored and used for detection and differentiation of sensor thermal aging and sensor element blackening. Thermal aging (also termed as de-graining effect) may occur in the oxygen sensor after multiple usages over time. Blackening may occur due to change in the material (e.g., Zirconia (ZrO2) into Zirconium (Zr)) present in the electrodes of the pump cell. A dark accumulation (e.g., of metallic Zr) may be observed on an electrode of the sensor, which may be termed as blackening. An alternating current (AC) voltage may be opportunistically or periodically applied to each of the pump cell and the Nernst cell of the oxygen sensor and the corresponding impedance of the cells may be estimated by measuring the respective pumping currents to generate a frequency scan. The impedance of each of the pump cell and Nernst cell may reduce due to heating over time. Thermal aging may be detected based on the change in impedance over time. However, after a period of time and after multiple usages, the change in impedance (observed in both pump and Nernst cell) due to thermal aging may stabilize. Further application of voltage to each of the cells may not show any further reduction in impedance. If there is blackening present in an electrode of the pump cell, after stabilization of thermal aging, further application of an AC voltage of higher peak voltage value may show a significant change in pump cell impedance. The extent of blackening may be estimated from the amount of change observed in the pump cell impedance relative to a previous measurement of pump cell impedance (such as during a last scan). Also, upon application of the same AC voltage to the Nernst cell, there may not be any further change in impedance, further confirming that a change in resistance observed in the pump cell is due to blackening and not from any further thermal aging. Following the detection of thermal aging, a compensation factor used during oxygen estimation using the sensor may be updated. In comparison, upon detection of sensor blackening, a lower target (reference) voltage and a conservative ramp rate for reference voltage application may be used during future oxygen estimation. Further, a diagnostic code (flag) may be set to notify the user of the degradation.
In this way, a change in impedance (or resistance) in each of the pump and the Nernst cell over time may be used to differentiate between thermal aging effects and element blackening in an oxygen sensor. By actively monitoring impedances of each of the pump and the Nernst cell following an application of alternating voltage, over time, it is possible to detect element blackening with higher certainty and better differentiate between the two forms of UEGO sensor degradation. The technical effect of differentiating between thermal aging effects and element blackening in an oxygen sensor is that appropriate corrective measures may be employed accordingly. By updating a corrective factor to compensate for thermal aging effects, accuracy of oxygen estimation may be maintained as a sensor ages. In comparison, when element blackening is detected, further damage to the sensor due to blackening may be limited by taking preventive measures such as applying a lower target voltage and a conservative ramp rate of reference voltage during oxygen estimations. Overall, by effective detection and differentiation of thermal aging and element blackening, accuracy and reliability of oxygen sensor operation is increased, enabling engine performance to be maintained.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.